As devices are scaled to sub-micron dimensions, formation of reliable, sub-micron interconnections (interconnects) becomes increasingly important. Also important for sub-micron devices is the use of planarization technologies during interconnect and wiring formation, as well as other stages of device formation. Many current processes used to form interconnects are unable to form interconnects with a sub-micron width in production. These processes fail for one or more reasons as described below.
A common method of forming an interconnect includes the steps of: 1) forming a patterned dielectric layer having contact openings to at least one underlying layer; 2) depositing a metal layer over the patterned dielectric layer; 3) forming a patterned photoresist layer over the metal layer; 4) etching the metal layer to form the interconnect; and 5) removing the patterned photoresist layer. This method has several problems. First, the metal layer typically has poor step coverage that causes the metal to be thinner along the walls of the contact openings. The thin metal increases resistance and may break when the device is stressed, causing an open circuit. Second, there may be problems with etching. Wet chemical etchants etch isotropically and generally give insufficient dimensional control for sub-micron devices. Dry etching is typically used, but some metal layers, such as copper and gold, are difficult to plasma etch. Third, aluminum can be plasma etched, but it also has problems. The substrate is typically alloyed or subjected to at least one heat cycle after the aluminum has been deposited and patterned. The heat cycle may cause the aluminum to spike into the silicon substrate lying at the bottom of a contact opening. Also, aluminum can form hillocks during an alloy or other post-metalization heat cycles. The hillocks may protrude from the interconnect to make electrical connection between adjacent interconnects, thereby shorting the interconnects together. Fourth, even if the interconnects may be formed without any of the previous problems, a subsequent dielectric layer that covers the interconnects typically has step coverage problems between narrowly spaced interconnects or requires a planarization process sequence.
Selective Electroless Metal Deposition (SEMD) is a method capable of forming an interconnect with a sub-micron width. A dielectric layer is deposited and patterned to form a patterned dielectric layer having an interconnect channel. As used in this specification, an interconnect channel is a pattern within a dielectric layer, wherein part of the dielectric layer is etched away. The interconnect is subsequently formed within the interconnect channel. Before depositing the metal using SEMD, the surface upon which the metal is to be deposited typically needs a treatment within the interconnect channel, so that metal deposits within the interconnect channel but not on the patterned dielectric layer. A common technique for treating the surface includes very heavily doping the interconnect channel with silicon ions at a dose of at least 1.times.10.sup.16 ions/cm.sup.2. Another treatment method includes contacting the surface with an activating solution. Surface treatments typically involve at least one additional processing step and may cause processing complications. Surface treatments may not be completely effective, such that metal does not properly deposit within the interconnect channel. In addition, particles and other foreign materials may lie on the dielectric layer and act as nucleating sites during the metal deposition. Therefore, metal particles are typically formed on the patterned dielectric layer. The metal particles may cause electrical shorts or other defects within the device. The inconsistent effectiveness and complications of the surface treatments steps and the formation of metal particles have prevented the SEMD method from being used in a production mode.
Dual damascene is another method of forming interconnects within interconnect channels. A single dielectric layer is deposited and patterned using a two-step etch process. The first step etches most of the dielectric layer within contact openings and the second step etches the interconnect channels and the rest of the dielectric layer within the contact openings. The depth of the interconnect channels are difficult to control because of film deposition and etch nonuniformities. The interconnect channels may be too deep in the center of the wafer and too shallow near the edge of the same wafer, resulting in large variations of interconnect resistance across the wafer. Metal deposition is complicated because the contact openings may have an aspect ratio of 2:1, 3:1, or more. The high aspect ratio makes sputter depositions virtually impossible. A metal layer may be deposited by chemical vapor deposition within the contact openings and interconnect channels. However, widely used interconnect materials such as aluminum, copper, gold, and silver are not typically deposited by chemical vapor deposition in production. Polysilicon and tungsten may be deposited by chemical vapor deposition, but these materials have higher resistivities compared to aluminum, copper, gold, and silver and are not generally used as interconnect materials. The SEMD method may be used but has the previously discussed problems. In addition, the SEMD method may form a metal void if metal deposited within the interconnect channels seal off their underlying contact openings before the contact openings are filled.
Copper is typically not used as an interconnect material. Although copper has a relatively low cost and low resistivity, it has a relatively large diffusion coefficient into silicon dioxide and silicon. Copper from an interconnect may diffuse into the silicon dioxide layer causing the dielectric to be conductive and decreasing the dielectric strength of the silicon dioxide layer. Copper interconnects should be encapsulated by at least one diffusion barrier to prevent diffusion into the silicon dioxide layer. Silicon nitride is a diffusion barrier to copper, but the prior art teaches that the interconnects should not lie on a silicon nitride layer because it has a high dielectric constant compared to silicon dioxide. The high dielectric constant causes an undesired increase in capacitance between the interconnect and other interconnects and the substrate.